1. Technical Field of the Invention
The present invention relates generally to an electromagnetic isolation apparatus for electronic equipment such as a portable telephone, a cordless telephone, or a selective calling receiver which is designed to prevent undesired electromagnetic waves or noise from entering or leaking out of the electronic equipment.
2. Background Art
FIG. 5 shows a radio terminal unit having a conventional isolation structure which includes a shield case 1, a control circuit substrate 2, a radio circuit substrate 4 having mounted thereon radio communication circuit components 3 such as a transmitter and a receiver, a printed grounding conductor 5, and grounding terminals 6.
The control circuit substrate 2 has mounted thereon control circuit components and is attached to an upper wall of the shielding case 1. The shield case 1 is made of a molded resin case coated or plated with metal. The radio circuit substrate 4 is attached to the bottom of the shield case 1. This type of isolation structure is disclosed, for example, in Japanese Patent First Publication No. 4-215500.
The printed grounding conductor 5 is formed on the radio circuit substrate 4 and is connected to the shield case 1 through the grounding terminals 6. Each of the grounding terminals 6 has an elastically deformable conductive strip 7 engaging an inner wall of the shield case 1. Such a structure is disclosed, for example, in Japanese Utility Model First Publication
The above isolation structure, however, has the following drawback. The substrates 2 and 4 are both thin, and the shield case 1 is made from resin material so that they are subject to setting or deformation. The attachment of the deformed substrate 2 or 4 to the shield case 1 will develop clearances therebetween through which undesired electromagnetic waves enter or leak out. In order to avoid this drawback, the prior art thus made it necessary to manufacture the substrates 2 and 4 and the shield case 1 with high accuracy, but such an approach results in an increase in manufacturing cost.
The elastically deformable conductive strips 7 of the grounding terminals 6 disposed between the radio circuit substrate 4 and the shield case 1 urge the substrate 4 to move away from the shield case 1, thereby increasing the clearance between the substrate 4 and the shield case 1. This causes more undesired electromagnetic waves to enter or leak out through the clearance, leading to malfunctions of, for example, the electronic components 3 or external circuits placed near the radio terminal unit.
The above isolation structure also uses two printed circuit boards, i.e., the control circuit substrate 2 and the radio circuit substrate 4, for improving the isolation performance. This type of printed circuit board is usually passed through a reflowing heater to mount thereon a plurality of electronic components. Different shapes of printed circuit boards encounter difficulty in passing through the same heater, however, thus requiring use of printed circuit boards of the same size or of different reflowing heaters, one for each printed circuit board. This results in an increase in design limitation or investment in plats and equipment.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide an improved isolation structure for an electronic device which is designed to isolate electronic components in the electronic device electromagnetically from the outside for preventing undesired electromagnetic waves from entering and leaking out of the electronic device.
According to one aspect of the present invention, there is provided an electromagnetic isolation apparatus for an electronic device having a casing within which a printed circuit board is mounted comprising: (a) a first shield case disposed within the casing of the electronic device to cover at least a portion of a first surface of the printed circuit board; (b) a second shield case disposed within the casing of the electronic device to cover at least a portion of a second surface of the printed circuit board opposite to the first surface; and (c) an elastic member interposed between an inner wall of the casing of the electronic device and a surface of one of the first and second shield cases so as to produce an elastic pressure urging the one of the first and second shield cases into engagement with a corresponding one of the first and second surfaces of the printed circuit board.
In the preferred mode of the invention, at least one of the first and second shield cases defines an enclosed chamber along with a corresponding one of the first and second surfaces of the printed circuit board within which an electronic component mounted on the printed circuit board is isolated electromagnetically.
The first and second shield cases are attached to each other to define an enclosed chamber surrounding at least a portion of each of the first and second surfaces of the printed circuit board.
Each of the first and second shield cases has a wall and side walls surrounding the wall to define a box-like conductive casing. Edge portions of the side walls of the first shield case engage edge portions of the side walls of the second shield case to define a chamber which surrounds the periphery of at least a portion of the printed circuit board.
A second elastic member is further provided which is interposed between a second inner wall of the casing of the electronic device opposite to the inner wall of the casing on which the elastic member is provided and a surface of the other of the first and second shield cases so as to produce an elastic pressure urging the other of the first and second shield cases into engagement with the other of the first and second surfaces of the printed circuit board.
The elastic member urges the one of the first and second shield cases into engagement with the corresponding one of the first and second surface of the printed circuit board to establish conductive communication between a portion of the one of the first and second shield cases and a grounding conductor printed on the printed circuit board.
One of the first and second shield cases has a wall and ribs extending from the wall to define at least one enclosed chamber along with a corresponding one of the first and second surface of the printed circuit board for electromagnetically isolating an electronic component disposed within the enclosed chamber.
A conductive spring member is further provided which is interposed between one of the first and second shield cases and a corresponding one of the first and second surfaces of the printed circuit board to establish conductive engagement therebetween.
The conductive spring member produces an elastic pressure urging the one of the first and second shield cases away from the printed circuit board against the elastic pressure provided by the elastic member. The elastic pressure provided by the conductive spring member is smaller than that provided by the elastic member so as to bring a portion of the one of the first and second shield cases and a portion of the corresponding one of the first and second surfaces of the printed circuit board into constant engagement with each other.
The one of the first and second shield cases has a wall and side walls surrounding the wall. Inside the side walls, ribs are formed on the wall, the ribs defining an enclosed chamber along with the corresponding one of the first and second surfaces of the printed circuit board. The conductive spring member is interposed between at least one of the ribs and the corresponding one of the first and second surfaces of the printed circuit board.